The present invention relates to adhesive compositions for bonding low surface energy substrates such as polyolefins, particularly to two-part acrylic adhesive compositions.
An efficient, effective means for adhesively bonding low surface energy plastic substrates such as polyethylene, polypropylene and polytetrafluoroethylene (e.g., TEFLON) has long been sought.
One specific area where low surface energy polymer materials of this type find utility is for plastic pipe joints where a mixed adhesive is applied by conventional means using a dauber, brush, swab or other suitable applicator device. One specific application method is to inject the adhesive using one of several well-known and readily available mixing and dispensing devices to the injection pipe joints such as those disclosed in U.S. Pat. Nos. 3,920,787, 3,920,268, 4,523,779, 5,486,024, 7,341,285, and published applications US 2006/0191623, US 2010/0259040 and JP 05-346189, each of which is incorporated by reference herein. In injection pipe joints, an adhesive is injected via a hole in the socket into a gap between a socket and an inserted pipe end and allowed to set after the socket and pipe end have been initially fitted together.
One specific injection pipe design is disclosed in commonly assigned copending U.S. Provisional Application No. 61/585,733, filed Jan. 12, 2012, the entire content of which is incorporated by reference herein.
Acrylic adhesives have been found to have utility for bonding these types of pipe joint designs. See, for example, US 2002/0195453 A1, the entire content of which is incorporated by reference herein.
See also U.S. Pat. No. 6,806,330, the entire content incorporated by reference herein, wherein acrylic adhesives using organoborane amine complex initiators, including trialkylborane-amine radical initiators, were found to provide strong adhesion to polyolefin and fluorinated polymer substrates.
Examples of acrylic adhesives used for pipe fitting wherein an organoborane amine complex initiator, including a trialkylborane amine initiator, is employed with a polymerizable acrylic compound are found in U.S. Pat. No. 7,638,007 and US 2009/0090454, each of which is incorporated by reference herein in its entirety.
A specific example of an acrylic adhesive is disclosed in U.S. Pat. No. 5,935,711, the entire content of which is incorporated by reference herein. The acrylic adhesive is disclosed as comprising a trialkylborane-amine radical initiator in combination with an aziridine functional material wherein the two-part adhesive includes a first part comprising a solution of the organoborane polyamine complex and the aziridine-functional material, wherein the aziridine-functional material is not reactive with the polyamine and a second part comprising a polymerizable blend of alkyl acrylate monomer and alkyl methacrylate monomer. A commercial embodiment of this system does not utilize an unsaturated monomer in the first part, a feature which limits formulation options, and which necessitates more meticulous mixing at the time of use.
Although the organoborane amine complex systems provide good bond strengths with polyolefin substrates, in at least some cases that bond strength does not develop rapidly, requiring cure times of longer than 24 hrs, sometimes as much as 72 hrs, before use loading can be applied to the bond. Also, some organoborane amine systems are quite sensitive to polyolefin source, suggesting antioxidant identities or amounts may impact reactivity of the decomplexed organoborane with the polyolefin substrate. In the plumbing arts, knowledge of the polyolefin source is typically not present so it would be desirable to obtain formulations that have less variability as to polyolefin source and which can reliably achieve high bond strength within a reliable 24 hr turnover time. There remains a need in the art for adhesive compositions having good stability, short turnover time, easy mixing, provide less sensitivity to polymer source and improved adhesion to difficult to bond, low energy surface substrate such as those used for bonding pipe joints.